1. Technical Field of the Invention
The present invention relates to cellular telephone systems and, in particular, to a method for assigning frequencies to transceivers in cells of a cellular telephone system supporting analog and/or digital communications channels.
2. Description of Related Art
Cellular telephone systems divide a large service area into a number of smaller discrete geographical areas called "cells" each typically ranging in size from about one-half to about twenty kilometers in diameter. Each cell is at least contiguous and/or overlapping with multiple adjacent cells to provide substantially continuous coverage throughout the service area. A base station including a plurality of transceivers capable of operating independently on different assigned radio frequencies is provided for each of the cells. Via the transceivers, the base stations engage in simultaneous communications with plural mobile stations operating within the area of the associated cell. The base stations further communicate via data links and voice trunks with a central control station, commonly referred to as a mobile switching center, which functions to selectively connect telephone calls to the mobile stations through the base stations and, in general, control operation of the system.
Each cell is assigned use of a predetermined set of frequencies from the cellular frequency band for use in providing its control and voice/data (traffic) channels. The assignment is typically made in accordance with a certain frequency plan. The frequencies used for the control and traffic channels assigned to a given cell are preferably spaced apart from each other across the frequency spectrum of the cellular frequency band. This serves to minimize the instances and adverse affects of adjacent channel interference.
Because only a limited number of frequencies are available in the cellular frequency band, the same frequencies that are assigned to one cell are also assigned to (i.e., reused by) other cells in distant parts of the service area. Typically, adjacent cells are not assigned to use the same frequency by the frequency plan. Furthermore, the power levels of the signal transmissions on any given frequency are limited in strength so as to limit propagation beyond the cell area. The foregoing precautions serve to reduce instances of co-channel interference caused by reuse of that same frequency in a distant cell. It is further noted that careful power level and distance assignment also assists in reducing instances of adjacent channel interference.
In spite of the precautions taken by service providers in the frequency plan assignment for a frequency reuse cellular telephone system and in the regulation of system operation, it is known that instances of co-channel interference do occur. This interference may be affected by a number of factors including: terrain irregularities; radio propagation changes; fading; multipath propagation; reflection; existence of human and natural obstructions; the number of available transceivers per cell; and variations in demand. This interference often adversely affects system operation by, for example, degrading voice quality on the traffic channels or interfering with the transmission and reception of control signals on the control channels. Service providers accordingly monitor on a cell by cell basis for instances of adjacent channel and co-channel interference on the assigned frequencies, as well as for instances of relatively low interference on other frequencies, and in response thereto make requests for a revision in the frequency plan assignment of frequencies for that cell. Such a revision is often referred to in the art as a "proposal" for change.
Now that both digital, analog and dual mode systems are being implemented, the process for making and implementing a request for a revision in the frequency plan assignment is becoming more complicated. The primary reason for the added complication relates to the fact that certain frequencies in the cellular frequency band may be specified solely for use with analog control or traffic channels, while other frequencies are specified solely for use with digital control or traffic channels. Still other frequencies in the cellular frequency band may be specified for dual mode use. At the same time, the transceivers comprising the cell configuration of a cell may be assigned for either analog or digital control/traffic channel use. The heart of the problem resides in the fact that mode authorizations for the frequencies included in the proposal may not necessarily coincide with the mode capabilities of the transceivers. Accordingly, it is imperative that any revision in the frequency plan assignment take into account the specified modes of the frequencies within the proposal in the context of the specified operating modes of the transceivers for the cell configuration. Another problem lies in the fact that each cell evaluates interference, and makes requests for frequency plan assignment revisions by considering only its own needs, and failing to consider the effect of such assignment revisions on other cells within the network.
What is needed then is a method that validates from a frequency mode perspective the frequencies of a given proposal in view of the transceiver mode requirements of a given cell, and further coordinates the cell based requests for revision to the frequency plan assignment to the benefit of the each of the cells of the network rather than the benefit of just an individual cell.